The present invention relates to apparatus and methods for detecting defects in reflective material, such as weld splices in metal rolls or coils.
Weld splices in tin plated steel coils (rolled up sheets) pose a problem in the can making and other industries that run the material into presses, slitters, or other forming equipment. Welds in a steel coil are harder than the steel coil itself, and may damage tooling when fed into a press. The weld may also result in defective products, such as a weld running across a finished can lid. This would result in an area without protective lacquer, as well as being cosmetically unacceptable. In addition, it is often necessary to locate a weld in order to properly position metal, for example to avoid a weld in a steel drum when attaching a handle or adding a painted label.
Inductive eddy current based sensors are currently used as weld splice detectors, thickness gauges, and double sheet detectors. They can detect a lap weld (overlap) of sufficient mass to effect an inductive change. However, modern welding techniques have moved toward narrow lap splices and butt splices, which do not result in sufficiently changed inductance to detect the weld in this manner.
Vision systems comprising video cameras, frame grabbers, computers, and the like accomplish gray scale comparisons in an attempt to detect welds. These are prohibitively expensive for the task, and not reliable enough.
A need remains in the art for a reliable, inexpensive system for detecting welds and other imperfections in metals.
It is an object of the present invention to provide a reliable, inexpensive sensing system for detecting welds and other imperfections in metals. The present invention includes a multi-element solid state diffuse light source capable of illuminating a broad section of metal. Light from the metal surface is focussed by a lens and detected by a multiple element array of light detectors, which is masked by a narrow slit aperture. The combination of the diffuse light source, appropriate lens selection, narrow slit aperture, and multiple detectors allows for uninterrupted inspection of a moving sheet of metal, even with appreciable changes in distance and angle between the metal and the detector array.
The present invention utilizes a multi-element linear detector array capable of redundant inspections in order to reduce the probability of false accepts from, for example, spotty or inconsistent defects. Since a weld forms a generally dark, dull line, but may include shiny spots, a single detector would give a false positive if it detected a shiny spot within a weld. Similarly, the narrow slit aperture ensures that only light reflected from the weld itself is detected, rather than light from the shiny portions next to the weld. Thus false positives resulting from averaging the small amount of light reflected from the weld with the shiny portions are avoided.
The signals from the linear detector array are passed to a processor system, which analyzes the signals to determine when a weld is passing under the sensing system. The processor system also includes a self calibration feature, which provides an accurate reference signal to compare against the signal being analyzed. A color filter is used to prevent ambient light from interfering with the detection process.